1. Field of the Invention
This invention relates generally to failure analysis of semiconductor devices, and more particularly, to an apparatus which provides high image depth resolution in analyzing such devices.
2. Discussion of the Related Art
As is well known, in the event of failure of a semiconductor device, a non-destructive failure analysis is undertaking in order to gain an understanding of the nature of the failure, with the goal of taking appropriate steps to avoid such a failure in future devices. As is also well known, a large percentage of such failures are due to a short circuit within the device. Such a short circuit causes a high current to flow at that point in device.
In the past, a heat sensing apparatus has been used to sense increased heat caused by increased current flowing at the short-circuit. As an example, one can use the characteristics of an infrared imaging device, or the characteristics of a liquid crystal display failure analysis apparatus may be chosen so that the display changes from an opaque to a clear state above a chosen temperature, and vice versa. The display would then be capable of indicating a xe2x80x9chot spotxe2x80x9d in the semiconductor device being analyzed, which would in turn indicate a short circuit.
Current semiconductor devices contain many, for example 10 or more, layers of metallization. With the many layers of metallization and the very small device dimensions involved, at best only general localization of a fault can be achieved.
Recently, a magnetic field imaging microscope has been introduced for undertaking failure analysis of semiconductor devices. An example is the Magma C-1 Magnetic Field Imaging Microscope, manufactured by Neocera, Inc. This apparatus uses a substantially flat superconducting sensor which is which is capable of detecting magnetic field intensity and direction perpendicular to the plane of the sensor. As the magnetic field induced by an electric current increases with increased current level, the sensor detects a higher magnetic field in the area of higher current, i.e., for example, a short circuit. A magnetic field image is obtained by scanning the semiconductor device with the sensor, and a software program converts the image into an image of current flow. The image produced is compared to a current flow image of a properly functioning device, so that the location of a fault can be better determined.
While such an apparatus provides advantages over the infrared and liquid crystal display systems described above, problems exist regard to depth resolution, i.e., the ability to discern which of the many layers in the semiconductor device may contain the fault which is imaged.
Therefore, what is needed is an apparatus which is capable of undertaking failure analysis of a semiconductor device in an effective manner with improved depth resolution.
The present invention is a magnetic field imaging apparatus for sensing a magnetic field generated by current flowing in a semiconductor device. The apparatus includes a pair of sensing devices which may be focused at a chosen depth in a semiconductor device. The sensing devices may be movable so that they may be focused at different focal points. The apparatus may also include three or more sensing devices, which can be chosen to operate in pairs or all together to define a variety of focus depths in a semiconductor device.
The present invention is better understood upon consideration of the detailed description below, in conjunction with the accompanying drawings. As will become readily apparent to those skilled in the art from the following description, there are shown and described embodiments of this invention simply by way of the illustration of the best mode to carry out the invention. As will be realized, the invention is capable of other embodiments and its several details are capable of modifications and various obvious aspects, all without departing from the scope of the invention. Accordingly, the drawings and detailed description will be regarded as illustrative in nature and not as restrictive.